Fluid-operated brake actuator with internal valve

ABSTRACT

A fluid-operated brake actuator having a hollow actuator rod between a spring chamber of a spring brake housing and a spring chamber of a service brake housing incorporates a valve within the actuator rod. The valve isolates the two spring chambers from each other in normal operation and releases a vacuum in the spring brake housing spring chamber when the spring brake is activated. A separate one-way check valve mounted in a well of the spring brake housing permits fluid to exhaust from the spring chamber to atmosphere and prevents contaminants in the atmosphere from entering the spring chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to fluid-operated brake actuators for vehicles andmore particularly to service and spring brake actuators combined intandem and having a spring brake actuator rod.

2. State of the Prior Art

An air brake system for a vehicle such as a bus, truck or the liketypically includes a brake shoe and drum assembly which is actuated bymeans of an actuator assembly operated by the selective application of afluid such as compressed air. Conventional air brake actuators have botha service brake actuator for actuating the brakes under normal drivingconditions by the application of compressed air and an emergency orspring brake actuator which causes actuation of the brakes when airpressure has been released. The emergency brake actuator includes astrong compression spring which forces application of the brake when airis released. This is often referred to as the spring brake. Typically,the spring brake actuator is disposed in tandem with the service brakeactuator.

The spring brake actuator is typically divided into two chambersseparated by a rubber diaphragm and pressure plate, with the spring inone of the chambers acting between and end wall of the spring brakehousing and the pressure plate. When full pressure is applied to theopposite chamber, air pressure acting against the diaphragm and pressureplate compresses the spring. In many applications, a spring brakeactuator rod is held in a retracted position by a relatively smallreturn spring. In newer applications, the spring brake actuator rod isintegral with the pressure plate and held in a retracted position by theair pressure.

In both designs, the spring brake actuator rod thus does not affect thenormal operation of the brake. The service chamber is typically dividedinto two chambers by a diaphragm. Depressing the brake pedal duringnormal driving operation introduces compressed air into one of thechambers of the service brake actuator which, acting against thediaphragm, causes a service brake push rod in the opposite chamber to beextended and the brakes to be applied with an application forceproportional to the air pressure in the service brake actuator.

In the event of a loss of air pressure or an intentional exhaustion ofair from the spring brake actuator, the brake will be mechanicallyactivated by the force of the strong compression spring acting on thespring brake actuator rod which, in turn, acts upon the service brakepush rod to apply the brakes. Thus, the spring brake portion serves bothas a parking brake and an emergency brake.

In tandem actuator assemblies, the spring brake push rod typicallyextends from a chamber in the spring brake portion, through an aperturein a wall separating the spring brake actuator from the service brakeactuator, and into a chamber in the service brake portion. Because atleast one of the adjoining chambers is usually pressurized, a seal isprovided at the aperture around the push rod comprising one or moreO-rings positioned in annular channels in the wall around the aperture.

When pressure is released from the spring brake actuator, the spring anddiaphragm extend significantly, expanding the volume of the spring brakeactuator chamber containing the spring. Means must be provided forallowing air to enter the expanded volume of the chamber. Conversely,when the spring is retracted, and the volume of the chamber contracts,means must be provided for evacuating air from the chamber. In manyprior brake actuators, the chamber containing the spring is simply opento atmosphere through ports in the chamber housing. However, this allowsdirt, salt, moisture and other unwanted material to enter that chamberthrough the ports. With the advent of hollow actuator rods containingcaging tools, the presence of foreign material within the actuator rodhas become an increasing concern of brake designers.

.Iadd.European Patent Number 0 025 558 to Lechner has attemted toaddress this problem by providing a pressure relief valve in the springbrake chamber that permits air to and contaminants to be evacuatedtherefrom. Another valve located within a hollow actuator rod permitsthe exchange of air between the service brake and spring brakechambers..Iaddend.

.[.Some.]. .Iadd.Other .Iaddend.prior designs have attempted to addressthis problem by providing a breather tube between the push rod chamberof the service brake actuator and the spring chamber of the spring brakeactuator, as in the U.S. Pat. No. 4,960,036 to Gummer et al. In thisdesign, the push rod chamber of the service brake actuator must bevented to atmosphere for normal application of the brakes. Venting ofthe push rod chamber occurs either through a separate vent, or throughthe external opening carrying the push rod. Thus, the spring chamber ofthe spring brake actuator remains open to atmosphere, and is stillexposed to moisture, dirt and grime, albeit through a longer passagethan a fully vented spring chamber. This longer passage may also preventthe spring chamber from drying as effectively as a sealed or more fullyvented spring chamber.

SUMMARY OF THE INVENTION

.[.The brake actuator of the present invention overcomes theselimitations..].

.[.A brake actuator for a vehicle comprises a service brake housing anda tandem spring brake housing. A first moveable member, disposed withinthe service brake housing, divides the interior thereof into a firstservice brake chamber and a second service brake chamber. The member isreciprocally moveable therein in response to the delivery and exhaust ofpressurized fluid to the first service brake chamber. A brake actuatorpush rod extends from the second service brake chamber and operablyconnects to the first moveable member for actuation of a brake. A secondmoveable member, disposed within the spring brake housing, divides theinterior thereof into a first spring brake chamber and a second springbrake chamber. The second member is reciprocally moveable therein inresponse to the delivery and exhaust of pressurized fluid to the secondspring brake chamber. The spring brake housing has an apertureestablishing communication between the first spring brake chamber andatmosphere. A power spring, disposed in the first spring brake chamber,moves the second moveable member upon exhaust of fluid from the secondspring brake chamber. A hollow actuator rod has a proximal end mountedto and extending through the second movable member for reciprocalmovement with the second moveable member between an extended positionand a retracted position. A distal end of the actuator rod is disposedwithin the first service brake chamber in a position to move the firstmoveable member when the second moveable member moves..].

.[.In accordance with the invention, a one-way valve is mounted to thespring brake housing at the aperture so that fluid flow from atmosphereto the first spring brake chamber is restricted. The actuator rod isopen at its proximal and distal ends so that the first spring brakechamber is in fluid communication with the first service brake chamberthrough the actuator rod. A control valve mounts within the actuator rodand has an open mode where the first spring brake chamber is in opencommunication with the first service brake chamber. In a closed mode ofthe control valve, the first spring brake chamber is not incommunication with the first service brake chamber. Closing means placesthe control valve in the closed mode when the actuator rod is in theretracted position. Fluid is delivered to the first spring brake chamberfrom the first service brake chamber upon movement of the actuator rodto the extended position and the first spring brake chamber is isolatedfrom the introduction of fluid from atmosphere..].

.[.Preferably, the one-way valve comprises a plug having a shaftdisposed within the aperture, and a head on the shaft larger than theaperture and exterior of the spring brake housing. Biasing means biasesthe head into sealing abutment with the exterior surface of the springbrake housing at a predetermined force, whereby when fluid pressurewithin the first spring brake chamber exceeds the predetermined force,the head of the plug will move away from the exterior surface and fluidwill escape through the aperture to atmosphere. The biasing meanspreferably comprises at least one flexible projection, extending fromthe shaft at a point within the first spring brake chamber into abutmentwith the interior surface of the spring brake housing. The flexibleprojection is deformed so that the deformation of the projection tendsto urge the head of the plug into sealing abutment with the springhousing. Preferably, the predetermined force is one to two pounds persquare inch of gauge pressure. An annular ring on the head contacts thespring brake housing to form the seal..].

.[.Preferably, the control valve is mounted within the distal end of theactuator rod. A caging tool extends into the actuator rod through itsproximal end, and the closing means comprises a valve pin on the end ofthe caging tool adapted to push against a portion of the control valvewhen the actuator rod is in the retracted position..].

.[.The control valve preferably comprises a tubular body disposed withinthe distal end of the actuator rod. A piston chamber within the body hasan open first end and an open second end. A main piston is disposedwithin the piston chamber for reciprocal movement therein. The mainpiston has an open position where the first end of the piston chambercommunicates with the second end of the piston chamber, and a closedposition where the piston blocks communication between the first end ofthe piston chamber and the second end of the piston chamber. The openand closed positions correspond to the open and closed modes of thevalve, respectively..].

.[.A spring biases the main piston toward its open position. The mainpiston has a coaxial interior bore with an open distal end and a closedproximal end. The distal end is oriented toward the first end of thepiston chamber. At least one radial passageway extends from the interiorbore radially outwardly through the main piston body to an exit opening.The exit opening is outside of the piston chamber in the open position,and inside of the piston chamber in the closed position. The valve pinextends into the piston chamber through its second end and holds themain piston in its closed position when the actuator rod is in theretracted position. The piston chamber first end communicates with thepiston chamber second end through the piston interior bore and theradial passageway. This communication only occurs when the open controlvalve is in the open position..].

.[.An overpressure stop valve can be provided. The stop valve comprisesa pressure plate moveable within the main piston second chamber betweenan open position and a restricted position. A pressure plate biasingmeans biases the pressure plate toward the open position. The pressureplate abuts a sealing lip in the interior chamber to restrict flowthrough the interior chamber in the restricted position. The pressureplate is away from the sealing lip in the open position..].

.[.Preferably, the pressure plate comprises a float piston, the biasingmeans comprises a spring between the float piston and the closed end ofthe interior chamber, and the sealing lip comprises an annular shoulderon the inside surface of the interior chamber. The float piston can beprovided with a small restricted flow aperture therethrough..].

.[.Alternatively, the pressure plate comprises a spring metal diaphragmhaving a flow aperture therethrough. The sealing lip comprises anannular lip on an end of a coaxial stanchion within the interiorchamber. The biasing means comprises the diaphragm having a restingshape wherein the diaphragm flow aperture is away from the annular lip.The diaphragm also has a deformed shape under a predetermined pressuregradient across the diaphragm wherein the diaphragm contacts the annularlip and the diaphragm flow aperture is restricted by the stanchion..].

.[.Preferably, the actuator rod has an exterior face at its distal endhaving at least one groove extending radially outwardly from the openingat the actuator rod distal end to a radial edge thereof. The firstservice brake chamber has an air port adapted to be connected to asource of pressurized air. A channel in the service brake housing leadsfrom the air port to the groove, so that the opening at the distal endof the actuator rod remains in communication with the air port if thefirst moveable member is in contact with portions of the spring brakehousing between the opening at the distal end of the actuator rod andthe air port..].

.Iadd.According to one aspect of the invention, a brake actuator for avehicle includes a service brake actuator housing and a spring brakeactuator housing. Each housing has an interior space. A first movableseal is disposed within the spring brake actuator housing dividing theinterior space thereof into a first spring brake actuator chamber and asecond spring brake actuator chamber. A second movable seal is disposedwithin the service brake actuator housing dividing the interior spacethereof into a first service brake actuator chamber and a second servicebrake actuator chamber. An actuator rod is operably connected to thefirst movable seal and movable therewith for reciprocation between afirst position wherein the actuator rod is essentially within theinterior space of the spring brake actuator housing and a secondposition wherein the actuator rod extends into the interior space of theservice brake actuator housing. The actuator rod has a passagewayextending therethrough for permitting the first spring brake actuatorchamber to be in communication with the first service brake actuatorchamber through the actuator rod. A valve at the actuator rod passagewayis adapted to establish flow of fluid through the passageway only whenthe actuator rod moves from the first position toward the secondposition. An abutting surface connected to the spring brake housing isadapted to abut a portion of the valve when the actuator rod is in thefirst position..Iaddend.

.Iadd.According to a further aspect of the invention, the actuator rodhas proximal and distal ends. The proximal end is connected to the firstmovable seal and the distal end is disposed within the first servicebrake chamber. A caging tool extends into the actuator rod through theproximal end, and the abutting surface comprises an end of the cagingtool..Iaddend.

.Iadd.According to another aspect of the invention, the brake actuatorcontrol valve includes a valve body disposed within the distal end ofthe actuator rod and a valve chamber therewithin. The valve chamber hasan open first end and an open second end. A main piston is disposedwithin the valve chamber for reciprocal movement therein and has an openposition such that the first end of the valve chamber communicates withthe second end of the valve chamber, and a closed position such that themain piston blocks communication between the first end of the valvechamber and the second end of the valve chamber. The open and closedpositions correspond to the open and closed modes,respectively..Iaddend.

.Iadd.According to a further aspect of the invention, the valve includesa spring biasing the main piston toward its open position. The mainpiston has a coaxial interior bore with an open distal end and a closedproximal end. The distal end is oriented toward the first end of thevalve chamber. An annular groove is located in the valve body adjacentto the valve chamber first end. The annular groove is in fluidcommunication with the central bore of the actuator rod. At least oneradial passageway extends from the piston interior bore outwardlyradially through the piston to an exit opening. The exit opening is influid communication with the annular groove in the open position and isblocked from the annular groove in the closed position. The caging toolextends into the second end of the valve chamber to hold the main pistonin its closed position when the actuator rod is in the retractedposition. Thus, the valve chamber first end communicates with the valvechamber second end through the piston interior bore and the radialpassageway only in the open position..Iaddend.

.Iadd.According to a further feature of the invention, the main pistonhas a sealing lip at the distal end of the piston interior bore. Anoverpressure stop valve includes a sealing element movable within thevalve chamber between an open position and a restricted position. Aspring biases the sealing element toward the open position such that thesealing element abuts the sealing lip to restrict flow through theinterior bore in the restricted position, and the sealing element isspaced from the sealing lip in the open position. In one embodiment, thesealing element includes a float piston. The spring is mounted betweenthe float piston and the closed proximal end of the piston interiorbore. The the sealing lip comprises an annular shoulder and the floatpiston has a small coaxial aperture therethrough. In another embodiment,a stanchion extends coaxially from the proximal end within the pistoninterior bore. The sealing element comprises a spring diaphragm having aflow aperture therethrough. The diaphragm has a resting shape whereinthe diaphragm flow aperture is away from the stanchion and deformableunder a predetermined pressure gradient across the diaphragm such thatthe diaphragm contacts the stanchion to block fluid flow through thediaphragm flow aperture..Iaddend.

.Iadd.Preferrably, the actuator rod distal end has an exterior facedefined between the valve body second opening and a radial edge of theexterior face. A groove extends radially outwardly along the exteriorface from the valve body second opening to the radial edge. The firstservice brake chamber has an air port adapted to be connected to asource of pressurized air and a channel in the service brake housingleading from the air port to the at least one groove. With thisarrangement, the valve body second opening remains in communication withthe air port when the first moveable sealing member is in contact withportions of the spring brake housing between the valve body second endand the air port..Iaddend.

.Iadd.According to another aspect of the invention, the spring brakehousing further includes a wall having an exterior surface exposed toatmosphere, an interior surface exposed to the first spring brakechamber, and an aperture therethrough. A one-way check valve is mountedto the wall at the aperture to permit fluid to flow from the firstspring brake chamber to atmosphere only when fluid pressure in the firstspring brake chamber exceeds a predetermined value. According to oneembodiment, the the check valve comprises a plug having a shaft disposedwithin the aperture. A head on the shaft is larger than the aperture andis located exterior of the spring brake housing. A biasing means isconnected to the plug for biasing the head into sealing abutment withthe exterior surface when fluid pressure within the first spring brakechamber is less than the predetermined value. The shaft extends into thefirst spring brake chamber and the biasing means can include at leastone flexible projection extending from the shaft toward and deformablyabutting the interior surface of the spring brake housing. Thedeformation of the projection urges the head of the plug into sealingabutment with the spring brake housing..Iaddend.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an air-operated brake actuator withvents and control valve of an isolation system according to theinvention;

FIG. 2 is an enlarged fragmentary cross sectional view of the vents ofFIG. 1;

FIG. 3 is an enlarged fragmentary cross sectional view of the controlvalve of FIG. 1;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 1;

FIG. 5 is an enlarged fragmentary cross sectional view of the controlvalve of FIG. 3, shown in the normal operating condition;

FIG. 6 is an enlarged fragmentary cross sectional view similar to FIG.3, showing an additional operating condition of the control valve ofFIG. 3;

FIG. 7 is an enlarged fragmentary cross sectional view of a secondembodiment of a control valve according to the invention;

FIG. 8 is an enlarged fragmentary cross sectional view of the controlvalve of FIG. 7 shown in the normal operating condition; and

FIG. 9 is an enlarged fragmentary cross sectional view similar to FIG. 3showing an additional operating condition of the control valve of FIG.7;

FIG. 10 is an enlarged fragmentary cross-sectional view of a thirdembodiment of a control valve according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a fluid-operated brake actuator 10 having a generalconfiguration well known in the art. The fluid-operated brake actuator10 comprises a service brake actuator 14 mounted in tandem to a springchamber or emergency brake actuator 16. A service brake push rod 12extends from the service brake actuator 14 for reciprocating movementbetween a retracted position and an extended actuating position relativeto the service brake actuator 14, and is provided with a clevis 17 whichis adapted to connect to a conventional brake shoe and drum (not shown)in a standard fashion. Reciprocating motion of the service brake pushrod 12 will cause the brake to be alternately applied and released.

The service brake actuator 14 comprises a cup-shaped service housingsection 18 and a double cup-shaped adapter housing 20 joined together bya clamp 22 to form a service brake chamber 23. The adapter housing 20 isalso sometimes known as a flange case.

A first elastomeric diaphragm 24 (also known as the service brakediaphragm) is suspended within the service brake chamber 23, theperipheral edge thereof secured in fluid tight enclosure between thecup-shaped service housing section 18 and the service side of theadapter housing 20 by the clamp 22. The first elastomeric diaphragm 24thus separates the service brake chamber 23 into two portions: a firstservice chamber portion 26 and a second service chamber portion 28. Thefirst service chamber portion 26 communicates with a source ofpressurized air (not shown) through an air service port 42 in theadapter housing 20. The second service chamber portion 28 is vented tothe atmosphere through at least one opening 32 in the cup-shaped servicehousing section 18. In FIG. 1, the first service chamber portion 26 isshown evacuated so that the first elastomeric diaphragm 24 is forcedagainst the adapter housing 20 because of the force from spring 46 inthe second service chamber portion 28.

The service brake push rod 12 extends through a central opening 30 inthe cup-shaped service housing section 18 and has a pressure plate 44 atthe end thereof within the second service chamber portion 28. Thepressure plate 44 bears against the first elastomeric diaphragm 24. Acompression spring 46 extends between the pressure plate 44 and theinterior surface of the cup-shaped service housing section 18. A pushrod guide 34 having an annular seat 39 is disposed within the centralopening 30 to guide reciprocal movement of the service brake push rod 12within the central opening 30 and also to receive the end of thecompression spring 46 and retain it in position around the centralopening 30. The compression spring 46 thus urges the pressure plate 44and the service brake push rod 12 to a fully retracted position asdepicted in FIG. 1.

To operate the service brake, compressed air is introduced through theair service port 42 into the first service chamber portion 26 to createa force against the first elastomeric diaphragm 24 and pressure plate 44sufficient to overcome the force of the compression spring 46, therebyextending the service brake push rod 12 toward the actuating position.The openings 32 permit rapid evacuation of air from the lower servicechamber section 23 as the service brake is actuated. Mounting studs 47are provided to mount the fluid-operated brake actuator 10 onto avehicle (not shown).

The spring chamber or emergency brake actuator 16 is defined by thespring side of the adapter housing 20 and a generally cylindrical head48 or spring chamber, which is clamped to the spring side of the adapterhousing 20 by a clamp 50 to form the spring brake chamber 51. A secondelastomeric diaphragm 52, known as the spring diaphragm, is suspendedwithin the spring brake chamber 51, the peripheral edge thereof securedin fluid tight enclosure between the cylindrical head 48 and the springside of the adapter housing 20 by the clamp 50. The second elastomericdiaphragm 52 thus separates the spring brake chamber 51 into twoportions: a first spring chamber portion 62 and a second spring chamberportion 63. The second spring chamber portion 63 is filled withpressurized air supplied through an air service port 54 in the adapterhousing 20 when the emergency brake is in its normal released positionas depicted in FIG. 1.

The adapter housing 20 includes a divider wall 35 which separates theadjoining service brake chamber 23 and spring brake chamber 51. A springbrake actuator rod 56, aligned with the service brake push rod 12, hasone end extending from the spring brake chamber 51 through a centralopening 38 in divider wall 35 for reciprocating motion through thecentral opening 38 between a retracted position and an actuatingposition. One or more O-ring seals 37 are provided in the centralopening 38 through which the spring brake actuator rod 56 reciprocates.

A distal end 55 of the spring brake actuator rod 56 terminates in areaction plate 66 in the first service chamber portion 26, and which isreceived in an annular seat 40 when the spring brake actuator rod 56 isin the retracted position as depicted in FIG. 1. An opposite, proximalend 57 of the actuator rod 56 extends through an opening 53 in thesecond elastomeric diaphragm 52 and terminates in a pressure plate 58which abuts an end of a large force compression spring 60. The pressureplate 58 abuts one end of the compression spring 60. A tubular portion61 of the pressure plate 58 extends generally axially along the axis ofthe compression spring 60. The tubular portion 61 of pressure plate 58is press-fit into the proximal end 57 of the spring brake actuator rod56 such that the pressure plate 58 and the spring brake actuator rod 56form an integral unit with the second elastomeric diaphragm 52 securedtherebetween.

During normal operation of the fluid-operated brake actuator 10, thespring brake actuator rod 56 will be in the fully retracted position, asdepicted in FIG. 1, by means of compressed air which is maintained inthe second spring chamber portion 63. When the compressed air isexhausted from the second spring chamber portion 63, the compressionspring 60, one end of which abuts the outer end wall of the cylindricalhead 48, forces the integral pressure plate 58 and spring brake actuatorrod 56 in the direction of the service brake push rod 12. The force ofthe compression spring 60 causes the spring brake actuator rod 56 to beextended through the central opening 38, thereby causing the reactionplate 66 to apply a force to the first elastomeric diaphragm 24 andpressure plate 44 of the brake actuator 14. This action causes theservice brake push rod 12 to be extended toward the actuating position,thereby applying the brake (not shown). When the spring brake actuator16 is to be released, compressed air is once again introduced into thesecond spring chamber portion 63 to a pressure sufficient to overcomethe force of the compression spring 60. The force of the compressed airagainst the second elastomeric diaphragm 52 causes the pressure plate58, the spring brake actuator rod 56 and the compression spring 60 to bereturned to the position depicted in FIG. 1.

In the embodiment shown, the spring brake actuator rod 56 is a hollowtube or rod provided with a central bore 68 to accommodate a brakerelease rod or caging tool 70. The central bore 68 of the spring brakeactuator rod 56 receives the caging tool 70, which passes throughaligned apertures 69, 73, and 75 in the cylindrical head 48, pressureplate 58 and spring brake actuator rod 56, respectively. The caging tool70 comprises a threaded elongated shaft 71, with one end having anenlarged head portion 72, which terminates in a valve pin 74. Theopposite end of the caging tool 70 is threaded through a head nut 76fixedly mounted to the cylindrical head 48, and has a hex head nut 78fixedly secured thereto.

The caging tool 70 primarily serves to enable manual retraction of thepowerful compression spring 60. Rotation of the hex head nut 78, threadsthe shaft 71 through the head nut 76, to axially move the caging tool 70with respect to the cylindrical head 48. The head portion 72 slidesfreely within the bore 68 of the actuator rod 56, yet is restrained byan inwardly directed annular flange 80 at the actuator rod proximal end57. Thus, withdrawal of the caging tool 70 by rotation of the hex headnut 78, causes the head portion 72 to abut the flange 80, and retractthe pressure plate 58 and spring 60. For safety, the spring 60 istypically retracted during repairs to the brake actuator 10 and duringshipping.

The cylindrical head 48 includes one or more ports 65 therein whichestablish communication between the first spring chamber portion 62 andthe atmosphere. A one-way vent 100, installed in each port 65, controlsthe flow of air through the port.

Turning now to FIG. 2, it can be seen that the vent 100 comprises amushroom shaped cap 102 external to the first spring chamber portion 62,a shaft 104 received within the port 65, and a plurality of flexibletabs 106 sloping outwardly from the shaft 104, from a point interior ofthe first spring chamber 62, toward the cap 102. The tabs 106 abut aninterior surface 108 of the cylindrical head 48 and bias the vent cap102 against an exterior surface 110 of the cylindrical head 48.

The vent 100 thus prevents air and contaminants in the atmosphere fromentering the first spring chamber portion 62 through the port 65. One ormore concentric rings 112 on the cap 102 contact the exterior surface110 of the cylindrical head 48 to enhance the seal formed between thecap 102 and the cylindrical head 48. Elevated pressure within the firstspring chamber portion 62 will overcome the biasing force of the tabs106 and vent cap 102 and allow air within the first spring chamberportion 62 to exhaust through the port 65. Preferably, the biasing forceof the tabs 106 will prevent the vent 100 from opening until thepressure within the first spring chamber portion exceeds one to twopsig, thereby maintaining a slight positive pressure within the firstspring chamber portion 62. It will be apparent that when the volumewithin the first spring chamber portion 62 expands upon release of thespring 60, air flow into the chamber from atmosphere through the ports65 will be blocked by the vents 100.

Returning to FIG. 1, air flow to the first spring chamber portion 62, inresponse to the extension of the spring 60, and diaphragm 52, comesthrough the actuator rod 56 and is controlled by a control valve 120 inthe distal end 55 of the actuator rod 56. The control valve 120 controlsthe flow of air from the first service chamber portion 26 into the firstspring chamber portion 62 in a manner to be more fully explainedhereinafter.

Turning to FIG. 3, the control valve 120 comprises a cylindrical body122 integrally formed with the reaction plate 66, and having acylindrical valve chamber 124 therein. The reaction plate 66 forms aclosed end 126 of the valve chamber; an opposite end 128 is open. Thecontrol valve body 122 is press fit into the distal end 55 of theactuator rod 56, and is sealed thereto by an o-ring seal 130 in acircumferential groove 132 about the valve body 122.

A main piston 134 is mounted within the valve chamber 124 for axialreciprocal movement therein. A spring 136, positioned between thereaction plate 66 and the main piston 134, biases the main piston 134away from the reaction plate 66. Outward movement of the main piston 134is restrained by a plurality of radially inwardly directed projections138 at the valve chamber's open end 128. An annular groove 140 isprovided in the wall of the valve chamber 124 immediately adjacent theprojections 138.

The main piston 134 comprises a cylindrical body 142, having a firstcylindrical interior wall 143 and an end wall 146, forming a cylindricalinterior chamber 144; and a second cylindrical interior wall 147 forminga larger diameter cylindrical outer chamber 148. Outer chamber 148 opensat one end to the valve chamber 124, and at an opposite end to theinterior chamber 144. A conical transition wall 150 separates the firstinterior wall 143 and the second interior wall 147.

A discoid shaped float piston 152 is disposed within the outer chamber148 for reciprocal movement and comprises a chamfered annular edge 154adapted to mate with the conical transition wall 150, and a small,axially extending, central aperture 156. A spring 158, positionedbetween the interior chamber end wall 146 and the float piston 152biases the float piston 152 away from the end wall 146. A plurality ofaxially inwardly directed projections 160 or a snap ring, or a washerrestrain the outward movement of the float piston 152.

A pair of annular grooves 162 in the outer surface of the main pistonbody 142 retain a pair of O-ring seals 164. A plurality of passageways166 extend radially outwardly from the interior chamber 144, through thefirst cylindrical wall 143, and exit the main piston body 142 at points165 between the annular seals 164. When the main piston 134 abuts theprojections 138 as shown in FIG. 3, the radial passageways 166 alignwith the annular groove 140, putting the central bore 68 of the actuatorrod 56 into fluid communication with the interior chamber 144 of themain piston 134.

Regardless of the position of the float piston 152 within the outerchamber 148, the interior chamber 144 communicates fluidly with thevalve chamber 124, and ultimately with the first service chamber portion26 (not shown in FIG. 3) through a central aperture 168 in the portionof the reaction plate 66 forming the valve chamber closed end 126. Whenthe chamfered edge 154 of the float piston 152 abuts and seals againstthe conical transition wall 150 (see FIG. 6), fluid flow into theinterior chamber 144 through the outer chamber 148 is possible onlythrough the small aperture 156. However, when the float piston 152 abutsthe inward radial projections 160 as shown in FIG. 3, flow may also passaround the float piston 152. Operation of the control valve 120, valvepiston 134 and float piston 152 will be more fully describedhereinafter.

Turning now to FIG. 4, a plurality of radial grooves 170 in the face ofthe reaction plate 66 lead from the central aperture 168 to a peripheraledge 172 thereof. A further groove 174 in the face of the wall 35, leadsfrom the annular seat 40 in the adapter housing 20 to the air serviceport 42 (FIG. 1). Thus, even when the first service chamber portion isevacuated and the first elastomeric diaphragm 24 is forced against theadapter housing 20, as shown in FIG. 1, the valve chamber 124 remains influid communication with the air service port 42 through the grooves170, 174.

Returning to FIG. 1, the control valve 120 controls the flow of air intothe first spring chamber 62 from the air service port 42, through thecentral bore 68 of the actuator rod 56, and apertures 75 and 73 in theproximal end 57 of the actuator rod 56 and pressure plate 58. Thecontrol valve 120 restricts or blocks flow under some operatingconditions and permits flow under other conditions.

A normal operating condition where the spring brake is not actuated isillustrated in FIG. 1 with the detail of the control valve 120 in thiscondition shown in FIG. 5. In this condition, the caging tool 70 isfully extended into the spring chamber 51. However, the second springchamber portion 63 is pressurized so that the diaphragm 52 and pressureplate 58 keep the spring 60 and actuator rod 56 retracted. Selectiveapplication of pressurized air to the first service chamber 26 extendsthe pushrod 12 to apply the brakes (not shown) in the normal manner.

Because the spring 60 and diaphragm 52 remain retracted, the firstspring chamber portion 62 is static and does not require an inflow ofair through the control valve 120. Hence, the control valve 120 is inthe closed position as shown in FIG. 5. An open control valve 120 wouldvent air every time pressurized air is introduced to the first servicechamber portion 26 to apply the brakes (not shown), by exhausting airthrough the aperture 168, the control valve 120, the central bore 68 ofthe actuator rod 56, into the first spring chamber portion 62 and out toatmosphere through the vents 100.

The valve pin 74 on the caging tool 70 .Iadd.has an abutting surface 74'which .Iaddend.extends into the valve chamber open end 128, pushing thevalve piston 134 toward the reaction plate 66, so that the radialpassageways 166 are no longer aligned with the valve chamber annulargroove 140. The radial passageways 166 abut the wall of the valvechamber 124, and the seals 164 prevent air from escaping from the pistoninterior chamber 144 into the upper portion of the valve chamber 124.The control valve 120 is thus closed and prevents air flow therethrough.

A second condition, when the spring brake is being applied, occurs whenneither the second spring chamber portion 63, nor first service chamberportion 26 are pressurized. When pressure is lost in the second springchamber portion 63, either due to a system failure or manual release,the spring 60 and the actuator rod 56 extend, thus moving the controlvalve 120 in the end of the actuator rod 56 away from the valve pin 74.This condition is illustrated in FIG. 3. Upon separation of the valvepin 74 from the end wall 146 of the control valve 120, the spring 136pushes the valve piston 134 into abutment with the radial inwardprojections 138, putting the radial passageways 166 into alignment withthe annular groove 140. The open control valve 120 allows air to flowfrom the air service port 42, through the control valve 120, through thehollow actuator rod 56 and into the expanding volume of the first springchamber 62. The spring 158 keeps the float piston 152 in abutment withthe inward projections 160, allowing air to freely flow past the floatpiston 152.

A third condition occurs when the service brake has been applied beforeactivating or releasing the spring brake. In the third condition (detailillustrated in FIG. 6), pressure is lower in the first spring chamberportion 62 than the first service chamber 26 (as when the driver setsthe parking brake by depressurizing the second spring chamber 63 whileapplying the service brake so that the first service chamber portion 26is pressurized). The resulting pressure differential across the floatpiston 152 forces the chamfered edge 154 of the float piston 152 intosealing abutment with the conical transition wall 150, preventing flowaround the float piston 152.

Any flow passing through the control valve 120 must pass through thesmall central aperture 156 in the float piston 152. The aperture 156allows some flow to fill the expanding volume of the first springchamber portion 62 as when the spring brake is being applied, forexample, but restricts excess flow which would otherwise exhaust toatmosphere through the vents 100. Of course, when the service brake isreleased, thereby removing pressure from the first service chamberportion 26, and relieving the pressure differential across the floatpiston 152, the float piston 152 reverts to the position shown in FIG.3.

As previously described with reference to FIG. 2, when the diaphragm 52and spring 60 are retracted, reducing the volume of the first springchamber portion 62, the vents 100 allow the excess air therein toexhaust to atmosphere. The one to two psig positive pressure within thefirst spring chamber portion 62 inhibits introduction of foreign matterfrom the brake actuator's service environment into the first springchamber portion 62 through the ports 65.

A second embodiment of a control valve 200, according to the invention,is illustrated in FIGS. 7 to 9 where like parts are numbered with likenumerals. Referring first to FIG. 7, the control valve 200 employs avalve body 122a substantially similar to the valve body 122 previouslydescribed with reference to the first embodiment control valve 120. Apiston 202, received within the valve body 122a for reciprocal movement,comprises a cylindrical body 204, having a cylindrical interior chamber206 with an end wall 208 and an open end 210, which opens into the valvechamber 124a. A short conical portion 211 of the interior chamber 206expands outwardly toward the open end 210. A plurality of radialpassageways 216 extend outwardly radially from the interior chamber 206to exit the piston body 204 at points in alignment with the annulargroove 140a when the piston 200 abuts the inward radial projections138a. A pair of annular grooves 212 about the exterior of the piston oneither side of the radial passageways 216 contain annular seals 214. Thepiston 200 functions similarly to the previously described firstembodiment piston 134.

An annular groove 218 in the interior chamber 206 at its open end 210retains a slightly convex spring metal diaphragm 220, having a centralaperture 222. A stanchion 224 extends coaxially in the interior chamber206 from the end wall 208 towards the metal diaphragm 220. A bore 226extending coaxially into the free end of the stanchion 224 forms anannular, axially extending lip 228.

Referring to FIG. 1, and to each of FIGS. 7 to 9, operation of thecontrol valve 200 in the three operating conditions of the brakeactuator 10 will now be described. When the brake actuator is in thefirst condition (normal operation, second spring chamber portion 63pressurized), as shown in FIG. 8, the valve pin 74a of the caging tool70a forces the piston 202 into the valve body 122a so that the radialpassageways 216 and seal 214 are inward of the annular groove 140a. Inthis position, the piston 202 closes the control valve 200.

When the brake actuator is in the second condition (second springchamber portion 63, and first service chamber portion 26 bothdepressurized) as illustrated in FIG. 7, the piston 202 abuts the inwardradial projections 138a, so that the interior chamber 206 communicateswith the central bore 68a of the actuator rod 56a. The interior chamber206 is open to the first service chamber portion 26 through the centralaperture 222 of the metal diaphragm 220. The metal diaphragm 220 andstanchion 224 perform essentially the same function as the previouslydescribed float piston 152. When the brake actuator 10 is in the thirdcondition (second spring chamber portion 63 depressurized and firstservice chamber portion 26 pressurized), as illustrated in FIG. 9, thepressure differential across the metal diaphragm 220 causes it to deformtowards the stanchion 224, engaging the lip 228 thereon. The centralaperture 222 of the metal diaphragm 220 aligns with the stanchion bore226 so that in this position flow through the aperture 222 is greatlyreduced. The spring constant of the metal diaphragm 220 is large, sothat pressure differentials above approximately 35 psig in the firstservice chamber portion 26 will cause the metal diaphragm 220 to deforminto contact with the stanchion 224.

An alternative means for restraining movement of the main piston 134 or134a is illustrated in FIG. 10, where like parts are numbered with likenumerals. The inwardly directed radial projections 138 or 138a of thefirst and second embodiments are replaced by a snap ring 300. The snapring 300 is received within an annular snap ring groove 302 in theannular groove 140b, and projects inwardly radially from the snap ringgroove 302 a sufficient distance to abut the end wall 146b of the mainpiston 134b. When the main piston 134b abuts the snap ring 300, theinterior chamber 144b communicates with the annular groove 140b throughthe radial passageways 166b. An annular groove 304 encircles theexterior of the control valve body 122b, at its uppermost edge. Theannular groove 140b remains in constant fluid communication with thecentral bore 68b of the actuator rod 56b through a plurality of axialpassageways 306 leading from the annular groove 140b to the annulargroove 304.

Preferably, the valve body 122, reaction plate 66, the main pistons 134,202, and the float piston 152 are formed of plastic.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure without departing from the spirit of theinvention which is defined in the accompanying claims. For instance, theinvention is not limited to the flexible plugs 100 described herein, andother means may be provided which pass air from the first spring chamber62 to atmosphere and block air flow from the operating environment intothe first spring chamber, such as a flap valve.

Also, alternate means can be provided in place of the control valves 120and 200 disclosed herein for preventing flow through the actuator rod 56when it is in its retracted position. For instance, the actuator rod 56can be made to seal against the end of the head 48 when the actuator rod56 is retracted, or the distal end 55 of the actuator rod 56 can beformed with a narrower diameter receiver which receives the head portion72 of the caging tool 70, so that the head portion 72 effectively sealsthe actuator rod 56. The invention has been described with reference toa diaphragm style spring brake actuator for illustration purposes only,and it would applicable to piston style brake actuators as well.

The embodiments for which an exclusive property or privilege is claimedare defined as follows:
 1. In a brake actuator for a vehiclecomprising:a service brake actuator housing and a spring brake actuatorhousing each having an interior space; a first movable seal disposedwithin the spring brake actuator housing dividing the interior spacethereof into a first spring brake actuator chamber and a second springbrake actuator chamber; a second movable seal disposed within theservice brake actuator housing dividing the interior space thereof intoa first service brake actuator chamber and a second service brakeactuator chamber; and an actuator rod operably connected to the firstmovable seal and movable therewith for reciprocation between a firstposition wherein the actuator rod is essentially within the interiorspace of the spring brake actuator housing and a second position whereinthe actuator rod extends into the interior space of the service brakeactuator housing; the improvement comprising: an aperture through thespring brake actuator housing at the first spring brake chamber and aone-way check valve at the aperture whereby the check valve allows fluidflow out of the first spring brake chamber and prevents fluid flow intothe first spring brake chamber through the aperture; the actuator rodhaving a passageway extending therethrough wherein the first springbrake actuator chamber is in communication with the .[.second springbrake.]. .Iadd.first service brake .Iaddend.actuator chamber through theactuator rod; .[.and.]. a .[.first.]. valve at the actuator rodpassageway adapted to establish flow of fluid through the passagewayonly when the actuator rod moves from the first position toward thesecond position.Iadd.; and an abutting surface connected to the springbrake housing adapted to abut a portion of the valve when the actuatorrod is in the first position.Iaddend..
 2. A brake actuator according toclaim 1 wherein the first moveable seal comprises a diaphragm.
 3. Abrake actuator according to claim .[.2.]. .Iadd.1 .Iaddend.wherein the.[.first.]. valve has an open mode wherein the .[.first.]. valve allowsfluid flow through the passageway, a closed mode wherein the .[.first.].valve inhibits fluid flow through the passageway, and a control means.Iadd.associated with the abutting surface and the valve .Iaddend.forplacing the .[.first.]. valve into the closed mode when the actuator rodis in the first position and for placing the .[.first.]. valve into theopen mode when the actuator rod is away from the first position.
 4. Abrake actuator according to claim 3 wherein .Iadd.the valve comprises avalve body having a valve chamber therewithin, the valve chamber havingan open first end and an open second end; and.Iaddend.the control meanscomprises a .[.bore wall defining a valve bore in the first valve, a.].piston .[.slidable.]. .Iadd.disposed .Iaddend.within the valve .[.bore,a channel through the piston and an abutting surface connected to thespring brake housing.]. .Iadd.chamber for reciprocal movement thereinand having an open position wherein the first end of the valve chambercommunicates with the second end of the valve chamber, and a closedposition wherein the piston blocks communication between the first endof the valve chamber and the second end of the valve chamber, the openand closed positions corresponding to the open and closed modes,respectively,.Iaddend. wherein the abutting surface abuts the piston andpushes the piston inwardly .Iadd.of the first end .Iaddend.of the valve.[.bore.]. .Iadd.chamber .Iaddend.when the actuator rod is in the firstposition .[.whereby the bore wall blocks the channel.]. .Iadd.to therebyclose the valve.Iaddend..
 5. In a brake actuator for a vehicle,comprising;a service brake housing having an interior space; a tandemspring brake housing having an interior space; a first movable sealingmember, disposed within the service brake housing, dividing the interiorspace thereof into a first service brake chamber and a second servicebrake chamber; a second movable sealing member, disposed within thespring brake housing, dividing the interior thereof into a first springbrake chamber and a second spring brake chamber; a power spring disposedin the first spring brake chamber between the spring brake housing andthe second sealing member, the power spring biasing the second sealingmember toward the service brake housing; and .[.a hollow.]. .Iadd.an.Iaddend.actuator rod having a distal end and a proximal end, theproximal end extending through the second sealing member for reciprocalmovement therewith between an extended position and a retractedposition, and the distal end disposed within the first service brakechamber; .Iadd.a central bore extending through the actuatorrod;.Iaddend.the improvement comprising: the spring brake housingcomprising a wall having an exterior surface exposed to atmosphere, aninterior surface exposed to the first spring brake chamber, and anaperture therethrough; a one-way check valve mounted to the wall at theaperture to permit fluid to flow from the first spring brake chamber toatmosphere only when fluid pressure in said first spring brake chamberexceeds a predetermined value; the actuator rod being open at itsproximal and distal ends wherein the first spring brake chamber is influid communication with the first service brake chamber through.Iadd.the central bore of .Iaddend.the actuator rod; a control valvemounted .[.to.]. .Iadd.within the distal end of .Iaddend.the actuatorrod for controlling fluid flow through the actuator rod and having anopen mode wherein the first spring brake chamber is in opencommunication with the first service brake chamber, and a closed modewherein the flow of fluid between the first spring brake chamber and thefirst service brake chamber is closed; .[.and.]. a valve actuatoroperatively connected to the control valve for placing the control valvein the closed mode when the actuator rod is in the retracted positionand for placing the control valve in the open mode as the actuator rodmoves from the retracted position toward the extendedposition.[...]..Iadd.; and a caging tool extending into the actuator rodthrough the proximal end, wherein the valve actuator comprises an end ofthe caging tool adapted to engage a portion of the control valve whenthe actuator rod is in the retracted position,.Iaddend. whereby fluid isdelivered to the first spring brake chamber from the first service brakechamber upon movement of the actuator rod toward the extended position..[.6. A brake actuator according to claim 5, wherein the check valvecomprises a plug having a shaft disposed within the aperture, a head onthe shaft larger than the aperture and exterior of the spring brakehousing, and biasing means connected to the plug for biasing the headinto sealing abutment with the exterior surface when fluid pressurewithin the first spring brake chamber is less than the predeterminedvalue..]..[.7. A brake actuator according to claim 6, wherein the shaftextends into the first spring broke chamber and the biasing meanscomprises at least one flexible projection extending from the shafttoward and deformably abutting the interior surface of the spring brakehousing wherein the deformation of the projection urges the head of theplug into sealing abutment with the spring brake housing..]..[.8. Abrake actuator according to claim 6, wherein the predetermined pressureis two pounds per square inch of gauge pressure..]..[.9. A brakeactuator according to claim 6 wherein the head has an annular ring whichcontacts the spring brake housing to form a seal..]..[.10. A brakeactuator according to claim 5 wherein the control valve is mountedwithin the distal end of the actuator rod..]..[.11. A brake actuatoraccording to claim 10 and further comprising a caging tool extendinginto the actuator rod through its proximal end, and wherein the valveactuator comprises an end of the caging tool adapted to engage a portionof the control valve when the actuator rod is in the retractedposition..]..[.12. A brake actuator according claim 5 wherein the springbrake housing further comprises:a wall having an exterior surfaceexposed to atmosphere, an interior surface exposed to the first springbrake chamber, and an aperture therethrough; and a one-way check valvemounted to the wall at the aperture to permit fluid to flow from thefirst spring brake chamber to atmosphere only when fluid pressure insaid first spring brake chamber exceeds a predetermined value..]..[.13.A brake actuator according to claim 12, wherein the main piston has asealing lip at the distal end of the interior bore, and furthercomprising an overpressure stop valve comprising:a sealing elementmovable within the piston chamber between an open position and arestricted position; and a spring biasing the sealing element toward theopen position; wherein the sealing element abuts the sealing lip torestrict flow through the interior bore in the restricted position, andthe sealing element is spaced from the sealing lip in the openposition..]..[.14. A brake actuator according to claim 13 wherein thesealing element comprises a float piston, the spring is mounted betweenthe float piston and the closed proximal end of the interior bore, andthe sealing lip comprises an annular shoulder..]..[.15. A brake actuatoraccording to claim 14 wherein the float piston has a small coaxialaperture therethrough..]..[.16. A brake actuator according to claim 14wherein a stanchion extends coaxially from the proximal end within theinterior bore, and the sealing element comprises a spring diaphragmhaving a flow aperture therethrough, the diaphragm having a restingshape wherein the diaphragm flow aperture is away from the stanchion anddeformable under a predetermined pressure gradient across the diaphragmwherein the diaphragm contacts the stanchion so that fluid flow throughthe diaphragm flow aperture is blocked by the stanchion..]..[.17. Abrake actuator according to claim 5 wherein the actuator rod has anexterior face at its distal end with at least one groove extendingradially outwardly from the opening at the actuator rod distal end to aradial edge thereof; andthe first service brake chamber has an air portadapted to be connected to a source of pressurized air and a channel inthe service brake housing leading from the air port to the at least onegroove, whereby the opening at the distal end of the actuator rodremains in communication with the air port when the first moveablemember is in contact with portions of the spring brake housing betweenthe opening at the distal end of the actuator rod and the airport..]..Iadd.18. In a brake actuator for a vehicle comprising: aservice brake actuator housing and a spring brake actuator housing eachhaving an interior space; a first movable seal disposed within thespring brake actuator housing dividing the interior space thereof into afirst spring brake actuator chamber and a second spring brake actuatorchamber; a second movable seal disposed within the service brakeactuator housing dividing the interior space thereof into a firstservice brake actuator chamber and a second service brake actuatorchamber; and an actuator rod operably connected to the first movableseal and movable therewith for reciprocation between a first positionwherein the actuator rod is essentially within the interior space of thespring brake actuator housing and a second position wherein the actuatorrod extends into the interior space of the service brake actuatorhousing;the improvement comprising: the spring brake housing comprises awall having an exterior surface exposed to atmosphere, an interiorsurface exposed to the first spring brake chamber, and an apertureextending through the spring brake housing; a one-way check valve beingmounted to the wall at the aperture to permit fluid to flow from thefirst spring brake chamber to atmosphere through the aperture only whenfluid pressure in the first spring brake chamber exceeds a predeterminedvalue and prevents fluid flow into the first spring brake chamberthrough the aperture; the actuator having a passageway extendingtherethrough wherein the first spring brake actuator chamber is incommunication with the first service brake actuator chamber through theactuator rod; a valve at the actuator rod passageway adapted toestablish flow of fluid through the passageway only when the actuatorrod moves from the first position toward the second position; and anabutting surface connected to the spring brake housing adapted to abut aportion of the valve when the actuator rod is in the firstposition..Iaddend..Iadd.19. A brake actuator according to claim 1wherein the actuator rod has a proximal and distal end, the proximal endconnected to the first movable seal and the distal end disposed withinthe first service brake chamber;a caging tool extending into theactuator rod through the proximal end, and wherein the abutting surfacecomprises an end of the caging tool..Iaddend..Iadd.20. A brake actuatoraccording to claim 19 wherein the valve is mounted within the distal endof the actuator rod..Iaddend..Iadd.21. A brake actuator according toclaim 5 wherein the control valve comprises:a valve body disposed withinthe distal end of the actuator rod and having a valve chambertherewithin, the valve chamber having an open first end and an opensecond end; and a main piston disposed within the valve chamber forreciprocal movement therein and having an open position wherein thefirst end of the valve chamber communicates with the second end of thevalve chamber, and a closed position wherein the main piston blockscommunication between the first end of the valve chamber and the secondend of the valve chamber, the open and closed positions corresponding tothe open and closed modes, respectively..Iaddend..Iadd.22. A brakeactuator according to claim 21 and further comprising: a spring biasingthe main piston toward its open position; the main piston having acoaxial interior bore with an open distal end and a closed proximal end,the distal end being oriented toward the first end of the valve chamber;an annular groove located in the valve body adjacent to the valvechamber first end, the annular groove being in fluid communication withthe central bore of the actuator rod; at least one radial passagewayextending from the piston interior bore outwardly radially through thepiston to an exit opening, the exit opening being in fluid communicationwith the annular groove in the open position, and being blocked from theannular groove in the closed position; and the caging tool extendinginto the second end of the valve chamber holding the main piston in itsclosed position when the actuator rod is in the retracted position;whereby the valve chamber first end communicates with the valve chambersecond end through the piston interior bore and the at least one radialpassageway only in the open position..Iaddend..Iadd.23. A brake actuatoraccording to claim 1, wherein the check valve comprises a plug having ashaft disposed within the aperture, a head on the shaft larger than theaperture and located exterior of the spring brake housing, and biasingmeans connected to the plug for biasing the head into sealing abutmentwith the exterior surface when fluid pressure within the first springbrake chamber is less than the predetermined value..Iaddend..Iadd.24. Abrake actuator according to claim 23, wherein the shaft extends into thefirst spring brake chamber and the biasing means comprises at least oneflexible projection extending from the shaft toward and deformablyabutting the interior surface of the spring brake housing wherein thedeformation of the projection urges the head of the plug into sealingabutment with the spring brake housing..Iaddend..Iadd.25. A brakeactuator according to claim 1, wherein the predetermined value of fluidpressure is two pounds per square inch of gaugepressure..Iaddend..Iadd.26. A brake actuator according to claim 23wherein the head has an annular ring which contacts the spring brakehousing to form a seal..Iaddend..Iadd.27. A brake actuator according toclaim 22, wherein the main piston has a sealing lip at the distal end ofthe piston interior bore, and further comprising an overpressure stopvalve comprising:a sealing element movable within the valve chamberbetween an open position and a restricted position; and a spring,biasing the sealing element toward the open position; wherein thesealing element abuts the sealing lip to restrict flow through theinterior bore in the restricted position, and the sealing element isspaced from the sealing lip in the open position..Iaddend..Iadd.28. Abrake actuator according to claim 27, wherein the sealing elementcomprises a float piston, the spring is mounted between the float pistonand the closed proximal end of the piston interior bore, and the sealinglip comprises an annular shoulder..Iaddend..Iadd.29. A brake actuatoraccording to claim 28, wherein the float piston has a small coaxialaperture therethrough..Iaddend..Iadd.30. A brake actuator according toclaim 27, wherein a stanchion extends coaxially from the proximal endwithin the piston interior bore, and the sealing element comprises aspring diaphragm having a flow aperture therethrough, the diaphragmhaving a resting shape wherein the diaphragm flow aperture is away fromthe stanchion and deformable under a predetermined pressure gradientacross the diaphragm wherein the diaphragm contacts the stanchion suchthat fluid flow through the diaphragm flow aperture is blocked by thestanchion..Iaddend..Iadd.31. A brake actuator according to claim 27wherein the actuator rod distal end has an exterior face defined betweenthe valve body second opening and a radial edge of the exterior face; atleast one groove extending radially outwardly along said exterior facefrom the valve body second opening to the radial edge; and the firstservice brake chamber has an air port adapted to be connected to asource of pressurized air and a channel in the service brake housingleading from the air port to the at least one groove, whereby the valvebody second opening remains in communication with the air port when thefirst moveable sealing member is in contact with portions of the springbrake housing between the valve body second end and the airport..Iaddend.